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I have never experienced this problem, but almost would bet it is some kind of leak at the intake manifold, especially since it is a new factor and you have checked almost all of the other essentials. An air leak after carb may be infrequent enough that most of us do not encounter the issue very often, and if that is it the problem should tend to diminish at higher rpms (increased air volume) because the flow through carb is greater enough to overcome some of the leak effects. I would start there. next.

I forget specifics of the trick, but have seen guys check for vac leaks (which this would be) with a small flame or light smoke - see where the flame or smoke is drawn to leak. If you find such a leak, try to stop it temporarily with some safe filler firm enough not to be drawn into the manifold cavern, and see if it solves the problem.

It manifold is leaking, better check plane of both manifold/head mating surfaces. Cheaper fix might be a fatter gasket. Once somebody told me about using JB Weld to plug such a leak, but don't think I would do that except as a very temporary fix.